This invention relates generally to improved, unitized in-line electron guns for color cathode ray tubes, and more specifically, to an improved gun electrode support structure that resolves main problems in gun assembly by reducing the incidence of fracture of the supporting beads, and by alleviating forces that can cause electrode displacement.
Unitized in-line electron guns generate three coplanar electron beams developed by thermionic emission of cathodes arranged in line. The resulting beams are formed and focused by a tandem succession of electrodes spaced along the central axis of the gun. The electrodes cause the beams to converge at multiple phosphor groups located on the faceplate of the color cathode ray tube. The prime objective of the design of such guns is to provide small spot size and enhanced resolution. To accomplish this objective, the electron gun electrodes and their field forming surfaces should be accurately spaced, the opposing faces of the electrodes should be parallel, and the beam passageways that extend from the point of beam origin at the cathodes in the base area of the gun and through to the convergence cup at the opposite end, should be coaxially aligned.
A standard production assembly procedure for such electron guns consists of the process of holding the discrete parts of the gun rigidly in proper relationship to each other by means of mechanical fixtures, then fastening all parts together by pressing an elongated heat-softened bead of glass onto support tabs or "claws" that project from the gun parts. These structural beads, also called "pillars" or "multiform beads", are typically made of glass of special composition and characteristics. Two elongated beads are normally used on unitized, in-line guns, one on each side of the succession of electrodes and extending in a direction parallel to the axis of the gun. Upon cooling of the beads and removal of the gun structure from the fixture, the gun parts are more or less permanently affixed in proper spatial relationship to each other, depending upon the stability and mechanical integrity of the bead-tab structure. The function of the beads is to provide mechanical support; retain proper positioning, spacing and coaxial alignment; and allow electrical isolation of the electrodes.
Production experience has shown this method to be less than ideal as evidenced by the many rejections of gun assemblies during manufacture because of fracture of the supporting beads upon cooling, or fracture during the high-voltage conditioning process. If such fractures do occur, the entire gun must be discarded even though it has reached the final stage of production and has accrued nearly all of its production cost. And there is another defect in this system that may occur: even if catastrophic fracture does not take place, the parts of the gun cannot be relied upon to remain permanently in proper spatial relationship or alignment because of the stresses that may have been built in by the beading may tilt or otherwise displace the electrodes, resulting in degeneration of gun performance.
The problems set forth are well known to practitioners in the field, and their solution seems to be a common one. The following prior art patents showing single-member electrode support systems seek to provide support for the electrode by making the individual electrode support members rigid: U.S. Pat. Nos. 2,788,966; 3,239,708; 2,950,406; 3,543,071; 3,701,920 and 3,614,502.
In U.S. Pat. No. 3,389,284, Andrews recites an electrode support structure for a delta-configured, three-gun color cathode ray tube in which the electrodes are supported by welded-on straps, the claw ends of which are embedded in three elongated glass beads. The claws have centrally located offset tines which deflect during a welding process to form an electrical contact with adjacent tines. So that they may deflect and yield easily without unduly stressing the grids or weakened support straps during the welding process, the tines are seakened at the point where they emerge from the straps. The weakened tines have no direct electrode support function.